Seven mixtures ranging from 5.2 to 12.5 percent by volume of natural gas in air were screened for their detonability using a bag test method. Erratic, uneven detonations were initiated at the 8.6 to 8.8 percent concentration level, with explosive charges ranging from 1001 to 1020 grams. At all other tested fuel concentrations deflagrations occurred. Although the detonations successfully propagated the entire length of the bag, a steady Chapman-Jouguet type wave front was not observed. The experimental detonation velocities and minimum initiator weight requirements are compared to those obtained in other studies under similar experimental conditions.